Weird Black "Shadows" On Rendered Image
A lot of your normals are facing inwards.  You probably can't see this because backface culling is disabled by default.  I always keep it activated so I can see these problems right away.  Here's what it looks like with Backface Culling enabled.



You can view the normals using the following setting.  

 

You can flip them with the following tool in Edit mode.



I also use remove doubles because there were many vertices that were overlapping which were not attached to each other.

The door is partially attached to the rest of the mail-box at the bottom which makes the polygon flow difficult to work with.  

Also, there is a shelf inside the model which is attached to the outer polygons, this may be intentional but has the same effect as interior faces: it causes rendering artifacts and was the biggest problem here.



Cycles cannot handle super low-poly models very well.  Basically triangles become very dark and visible at areas where the lights hit them at glancing angles.  Adding more geometry, lights and an HDR texture can help with this.  

I used inset on the top piece and some loop cuts along the rim.





The main box and the door have a Bevel Modifier attached and set to 'Angle'.

I applied the Solidify modifier on the door latch/hook thing and added inset and loop cuts to it as well as a sub-surface modifier to make the edges more crisp.





Here's the .blend http://www.pasteall.org/blend/34426